This invention relates to machine-tool boring bars in general, and to vibration damping devices for such boring bars in particular.
A pervasive problem in the high speed machining of the harder materials such as titanium, zirconium and their alloys is the tool vibration encountered, especially when boring holes in which the diameter is small compared to the depth of the bore. The present inventor has made several improvements in the design of boring bars for such work, with significant reductions in tool vibration and a consequent improvement in machining speed as the result.
Shurtliff U.S. Pat. No. 3,172,308 discloses a damped boring bar in which the damping action is accomplished by a spring-loaded damper body internally suspended in the boring bar and made massive by filling an internal cavity in the damper body with a heavy metal such as lead. Some damping action may also occur because the suspension of one end of the suspended damper body may permit minute lateral frictional movements in response to vibration. Adjusting the tension of the loading spring permits some compensation for machining variables.
Shurtliff U.S. Pat. No. 3,230,833 discloses an improvement in damped boring bars in which the minute lateral movement of the suspended internal damper body is encouraged by shaping the moving end of the internal damper body. Further damping effect is obtained by filling the central cavity of the suspended damper body with heavy metal particles, themselves suspended in a heavy viscous liquid, such as mercury. Shaping the suspended damper body so that most of its mass is located close to the cutting tool is also disclosed. Again, adjusting the tension of the loading spring compensates for a broadened range of machining variables--and thus vibration frequencies--over the previous disclosure.
Shurtliff U.S. Pat. No. 3,582,226 discloses yet another improvement, in which the suspended internal damper body, now shaped to place most of the damper mass as close as possible to the cutting tool, also contains additional internal dampers which depend for their efficacy on multiple minute lateral frictional movements. Because these additional dampers have differing masses, the range of vibration frequencies successfully damped is broadened even more over the prior disclosures. Adjusting the tension of the loading spring permits compensation for machining variables.
Shurtliff U.S. Pat. No. 3,601,229 discloses a further improvement over the above-referenced disclosures, in that the shaped, suspended internal damper body now incorporates a slidable cap on the massive end of the damper body. The movements of the slidable cap alternately enlarge and diminish a cavity filled with hydraulic fluid, this cavity being created by the space between the damper body and the cap. As a result of this alternate enlarging and diminishing of the cavity, hydraulic fluid is forced through small channels, giving a further hydraulic damping effect, in addition to the frictional and inertial damping inherent in the design. The range of vibration frequencies which can be damped successfully is increased substantially over the prior references, and adjusting the spring loading permits compensating for an increased range of machining variables.
One feature common to the above discussed references is that the damper body moves longitudinally as well as laterally, by the very nature of the design, and this may have certain undesirable effects. For example, although lateral tool vibration may be almost entirely eliminated, longitudinal vibration, because of the massive nature of the damper, may be present and may have an adverse effect upon the tool during very precise finishing operations.